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The goal of this exercise was to provide estimates of the mother-to-child transmission rate (TR) of human immunodeficiency virus type 1 (HIV-1), calculated according to standardized methods. Prospective cohort studies in Africa (8), the Caribbean (1), Europe (3), and the U.S.A. (1) observed from birth children born to women known to be HIV infected at the time of delivery. TRs were calculated and compared by investigators during a meeting in Ghent (Belgium) in September 1993 according to agreed methodology. TRs were calculated following the direct and the indirect methods developed in 1992 by the Ghent Working Group. The direct method uses a classification of children born to HIV-seropositive mothers according to their probable HIV infection status at 15 months of age or before, if they die or are lost to follow-up. Minimum, intermediate, and maximum estimates of TR are computed depending on how children classified as indeterminate are counted. The indirect method is applied for studies with a comparison cohort of children born to HIV-seronegative mothers. TRs in developed countries ranged from 14 to 25% with the direct method (intermediate estimate). In the developing world, they ranged from 13 to 42% with the direct method, from 21 to 43% with the indirect method, and most of the studies reported a TR in the range of 25 to 30%. With use of a standardized methodology, the overall TR of HIV-1 tends to be higher in Africa than in Europe or the U.S.A. The variation in TRs is probably due to differences in factors associated with increased risk of transmission. This is of importance for the design and implementation of trials evaluating interventions aimed at reducing mother-to-child transmission of HIV.